1. Field of the Invention
This invention relates generally to the field of manufacturing resin impregnated fabric articles and more specifically to the manufacture of such articles which are useful as dielectric substrates in printed circuit boards and to improvements in said manufacturing which avoid shrinking of the articles during final curing of the resin.
2. Prior Art
It has long been known to use resin impregnated fabric, more commonly known as prepreg, as a dielectric substrate in the manufacture of printed circuit boards.
In a typical process, the fabric, which may be made from any suitable material, is coated with a curable resin. The resin is then partially cured, or "B-staged", in order to provide substantially planar cards or boards of prepreg which can be used to construct a printed circuit board. Typically the prepreg comprises glass cloth impregnated with an epoxy resin as is shown in U.S. Pat. No. 3,660,199 to Riccitiello et al.
Often, prior to personalization of the printed circuit board, the prepreg components are laminated together to form thicker dielectric layers and are sometimes sandwiched between layers of a conductive material. The conductive layers are then sometimes covered with additional layers of prepreg to form buried conductive layers.
It is also typical in the manufacture of printed circuit boards having one or many layers to make holes, known as through holes, completely through the printed circuit board. The holes are made by a variety of techniques such as drilling or laser cutting. They may be plated with a conductive material for use in electrical communication between the sides of the printed circuit board or they may be used to receive pins of components which are plugged into the printed circuit board.
Finally the prepreg or the drilled laminate is fully cured or "C-staged". C-staging after lamination helps to improve laminar adhesion in printed circuit boards comprising layers of prepreg. The C-staged prepreg or laminate is then normally personalized on at least one side with an electrically conductive circuit, which may be applied by any of a variety of well-known techniques, such as silk screen printing or photomasking combined with plating and etching.
It has been a problem with prepreg made by the process described above, especially when B-staged by heating in a vertical curing oven, that the prepreg shrinks during C-staging. Such shrinking is more dramatic when the prepreg is B-staged in a vertical drying oven, although it is also noted when the prepreg is B-staged in a horizontal oven. It is believed that the weight of the resin coated web as it is pulled up the vertical curing oven or through a horizontal oven stresses the warp of the fabric. It is believed that during C-staging the forces exerted by the warp in trying to recover from this stress causes the prepreg to shrink in a direction parallel with the warp.
Such shrinking causes through holes drilled in the prepreg, or in a laminated substrate comprising such prepreg, to move toward the center of the board during C-staging.
Coping with this problem has been a major factor in the production of printed circuit boards. Generally the shrinkage is compensated for by the complex and imprecise technique of anticipating shrinkage, such as by making through holes in locations other than where they are ultimately desired in anticipation that they will move to the desired location.
There is a need for prepreg which demonstrates reduced shrinkage during C-staging.